In various computing systems, such as the eServer zSeries, S/370-XA, ESA/370 and ESA/390 systems, offered by International Business Machines Corporation, Armonk, N.Y., each system footprint, referred to as a central processing complex (CPC), is limited to a maximum configuration of 256 I/O paths, such as, for example, 256 channel paths. One of the reasons for the constraint is that the  unique identifier for each channel path configured to the CPC, referred to as the Channel Path Identifier (CHPID), is defined as an 8-bit binary number, which provides 256 unique CHPID values from zero to 255. Since this value is only 8-bits, only 256 paths with unique identifications are possible. One solution is to increase the size of the CHPID; however, this has serious consequences for the many programs that use the CHPID.
This 256-channel path limitation has restricted the ability to provide significant increases in the overall CPC in terms of the maximum processing capacity of the central processors provided by the CPC. Further, computing systems are being created in which the total computing capacity of the systems is increasing past the point where 256 channel paths are sufficient in order to provide adequate I/O bandwidth and I/O configuration flexibility necessary to fully utilize the increased numbers of central processors.
Thus, a need exists for a capability to extend the functionality of an input/output subsystem. In one example, a need exists for a facility that can provide more than 256 I/O paths in a manner that is minimally disruptive to the programs using the paths.